Photoractivating enzymes (PREs) play a unique role in DNA repair by their specific, light-dependent monomerization of cyclobutyl pyrimidine dimers in DNA. We shall determine the structure of E. coli PRE, both apoprotein and "cofactor", and study their relation to the binding and photolytic reactions. We shall isolate large quantities of the mammalian PRE and determine its structure. The specificity of the enzyme allows its use as a dimer probe: if detrimental biological effects of UV can be reversed in a true photoenzymatic reaction, dimers played a role in the production of that damage. Although mammalian tissues contain PRE, expression of the phr gene in cells in culture has stringent requirements. We have thus developed a method of polyethylene glycol insertion of E. coli enzyme into mammalian cells. This should allow extension of the "PR test" to cells with low intrinsic PRE levels (e.g. xeroderma cells, rodent cells) or to conditions unfavorable for PRE production. We are using an alkaline agarose gel technique for measuring dimers in (non-radioactive) human skin. We shall examine repair by excision and photoreactivation in human skin exposed to light from different sources; we shall obtain action spectra for dimer production in leukocytes, and in mouse and human skin.